Products including semiconductor devices, active matrix liquid crystal panels, solar cell panels and magnetic discs are fabricated by building up successively prescribed thin films on the clean surfaces of boards such as of silicone and glass. Manufacture of each product requires extremely high accuracy, and presence of any impurity even in a trace amount adhered or adsorbed on the board surface makes it difficult to give a high-quality product. Meanwhile, the boards can be electrified during transportation or storage or through various treatments to be liable to attract impurities contained in the ambient air and have them deposited thereon.
For example, if a board under manufacture has any moisture adsorbed on the surface thereof as an impurity, the moisture is causative of troubles in the manufacturing process. Further, if the amount of the adsorbed water is great and if the ambient air contains oxygen, a native oxide film is formed on the surface of the board to hinder formation of prescribed thin films thereon. Meanwhile, presence of any moisture on the surface of an insulating film such as of SiN.sub.x in a process for manufacturing thin film transistors (TFT) to be employed in liquid crystal panels prevents formation of an amorphous silicon (a-Si) film with a uniform thickness and accuracy.
In a step of forming a gate oxide film in the process for manufacturing integrated circuits (IC), if any moisture is present on the surface of n-portion or p-portion, an SiO.sub.x film is formed at the interface between SiO.sub.2 and Si, preventing the MOS transistor from functioning as a switch. Simultaneously, if any moisture is present on the surface of a capacitor, an SiO.sub.x film is formed at the interface thereof to prevent charging against a capacitor electrode, and the capacitor fails to function as a storage element.
Furthermore, in a wiring step, while a TiN film is formed for preventing spiking to be caused by tungsten silicide before formation of a tungsten (W) film, presence of any moisture on the board surface causes troubles including drop in the adhesion of the TiN film. In addition, if heat treatment and the like is carried out in the presence of impurities other than moisture, e.g., in the presence of CO or CO.sub.2, the carbon reacts on the surface of the board (Si) with the silicon to form an SiC film, causing troubles in the performance characteristics of the device.
Various kinds of equipment employed for manufacturing products including semiconductor integrated circuits are generally installed in a particle-free clean room. Since boards used for manufacturing such products must undergo many treatment steps, they are carried from one treatment step to another treatment step by a transportation system. Since this system is also located in the clean room, the boards under transportation are exposed to the air in the clean room (clean room air). The transportation system is provided with a conveyor for transporting boards and a storage for storing the boards temporarily. The storage further contains a carriage for moving the boards inside.
The inside of the clean room is usually maintained at a temperature of 20 to 25.degree. C. and at a relative humidity of 50%, and large amounts of impurity gases are present, although particles are removed. Accordingly, the impurities present in the clean room air are adsorbed on the surface of the boards. For example, moisture is adsorbed at once on the surfaces of boards. It is actually difficult to remove moisture in the clean room completely so as to prevent such moisture adsorption.
Under such circumstances, Japanese Unexamined Patent Publication No. Hei 10-163288 discloses a transportation robot. This robot carries boards in a pocket charged with dry nitrogen or synthesized air from one treatment step to the next treatment step. According to this technique, boards are prevented fully from being brought into contact with the clean room air during transportation from the transportation system to a treating equipment or equipment. Meanwhile, Japanese Unexamined Patent Publication No. Hei 5-211225 discloses a system in which equipments are connected with tunnels purged with an inert gas such as nitrogen gas, and boards are transported in the inert gas tunnels. According to this system, boards can be transported without exposure to the ambient air, and the boards can be transported utilizing the flow of the inert gas. Further, Japanese Unexamined Patent Publication No. Hei 8-191093 discloses a concept as an improvement of the above system, in which the nitrogen gas flowed through the inert gas tunnel is introduced to a cryogenic separation type nitrogen generator for reutilization therein. Further, WO 97-24760A1 discloses a transportation method utilizing a mixed gas of an inert gas and oxygen as security countermeasures.
Here, clean dry air produced according to the conventional method is obtained by pressurizing the ambient air used as a raw material by an air compressor to a predetermined pressure before introduction to a catalytic purification unit and an adsorptive purification unit. Accordingly, the unit cost of producing the clean dry air is expensive compared with that of the conventional clean room air. On the other hand, there is required a huge quantity of clean air for transportation or storage of boards. For example, in the case where 8-inch Si boards are treated 1000 pcs/day in a semiconductor integrated circuit manufacturing plant, the quantity of clean air used therefor amounts of about 2,000,000 m.sup.3 /h.
Therefore, it is a great economic load to use large amounts of expensive clean dry air. Further, the cost of the plant for producing such amount of clean dry air amounts to several tens of billion yen per plant, so that it is actually difficult to construct such a scale of plant. In order to reduce the scale of the plant, while there is considered reutilization of the used clean dry air, this requires a recycling fan which can circulate a huge amount of used clean dry air and which does not cause pollution of air and a material of the recycling passage which forms no contaminant.
However, use of a recycling fan requires power for driving it, leading elevation in the cost for achieving recycling of air. In addition, the recycling fan is difficult to achieve recycling of air without causing air pollution since it employs oil for lubrication of bearings in many cases.
Further, if pipes made of galvanized steel plates are used as the recycling passage for the used clean dry air, it can happen that oxygen contained in the recycling air reacts with hydrogen diffusing from the inside of the galvanized steel plates to form moisture on the inner surfaces of the pipes. Accordingly, it is necessary to install additionally an impurity removing apparatus in order to remove contaminants occurring from the recycling fan or the piping material of the recycling passage.
Further, in case of any trouble occurred in the clean dry air producing plant, the plant itself is held up, so that a great investment should be made in backup equipment. Meanwhile, in clean dry air-consuming equipments, since large amounts of clean dry air have been introduced thereto with almost no consideration of the concentrations of impurities contained in the clean dry air, ambient control cannot be achieved effectively, and the clean dry air is consumed wastefully.